1. Field of the Invention
The present invention relates to a method for controlling dynamic wavelength allocation, and more particularly to such a method in an optical line terminal in an optical communications network system formed by one or more optical network units together with the optical line terminal, which includes a plurality of optical subscriber units. The present invention also relates to such an optical line terminal.
2. Description of the Background Art
A communication network that connects telecommunications carrier's equipment to subscribers' equipment is called an access network. With the increase in communication traffic in these days, among access networks, optical access networks become predominant, which rely upon optical communication to allow a huge amount of information to be transmitted.
One type of optical access networks is a passive optical network (PON). Generally, the PON system includes a single optical line terminal (OLT) provided in a common carrier's premises, a plurality of optical network units (ONUs) provided in subscribers' premise, and an optical splitter. The OLT and ONUs are connected with the optical splitter by optical fiber lines.
A single-core optical fiber line is used to connect the OLT with the optical splitter. The single-core fiber line is split by the optical splitter and shared between the plurality of ONUs. The optical splitter is an inexpensive passive device. Thus, the PON system is economical and easy to maintain. For those reasons, PONs become rapidly introduced.
In the PON system, signals directed from ONUs to the OLT, i.e. upstream optical signals, are combined by the optical splitter and transmitted to the OLT, while a signal directed from the OLT to the ONUs, i.e. a downstream optical signal, is split by the optical splitter into signals, which are in turn transmitted to the ONUs. In order to prevent interference between upstream and downstream optical signals with each other, the upstream and downstream optical signals are allocated to wavelengths different therebetween.
Various types of multiplexing schemes are used in PONs. The multiplexing schemes used in PONs include time division multiplex (TDM), which allocate time slots different on the time axis to subscriber terminals, wavelength division multiplex (WDM), which allocates different wavelengths to subscriber terminals, and code division multiplex (CDM), which allocates different codes to subscriber terminals. Among those multiplexing schemes, PONs using the TDM, i.e. TDM-PONs, are currently most widely used. In TDM-PONs, time division multiple access (TDMA) is predominantly used. According to the TDMA, control is made such that an OLT manages the timing of transmission from ONUs so as to prevent upstream optical signals from colliding with each other between those ONUs.
A sort of PON system that uses Ethernet (trademark) is called Ethernet-PON, and a PON system that uses Gigabit (1×109 bit/sec) Ethernet is called GE-PON. The GE-PON is standardized by IEEE (The Institute of Electrical and Electronics Engineers, Inc.) 802.3 ah and IEEE 802.3 av, for example.
Optical access networks may employ cyclic sleep, in which ONUs that are not involved in communications are periodically placed in sleep mode in order to reduce power consumption in the systems, as taught by Yukio Hirano, et al., “A Study of Link Monitoring Methods during ONU Power Saving on PON system” Communication Society Conference of The Institute of Electronics, Information and Communication Engineers, September 2011, B-8-1. The cyclic sleep will be described below with reference to FIG. 1. FIG. 1 is a sequence diagram illustrating the cyclic sleep. An example will be described in which the first one of plural ONUs enters its sleep mode.
The cyclic sleep proceeds in the following fashion. The first ONU which stays in no upstream traffic condition transmits a sleep request (RQ) signal 600 to an OLT. The OLT, when having received the sleep request signal 600, determines that downstream traffic to the first ONU has not occurred in this example, and transmits a sleep permission signal 602 to the first ONU. The first ONU receives the sleep permission signal 602 to enter its sleep mode 604.
The sleep permission signal 602 contains information on activation time at which the first ONU entering its sleep mode is to be activated. When the activation time comes, the first ONU in its sleep mode becomes reactivated. The OLT transmits a GATE message 606 to the reactivated first ONU. The first ONU receives the GATE message 606. If the first ONU determines that upstream traffic has not occurred, it transmits a REPORT message 608 to the OLT and enters its sleep mode 604 again. Then the OLT periodically transmits a GATE message 606 to the first ONU. The first ONU is periodically activated from its sleep mode 604 at the GATE message transmission intervals. The first ONU receives a GATE message 606 and transmits a REPORT message 608 in a period 610 during which the first ONU is active. In this way, in the cyclic sleep, an ONU in its sleep mode is periodically awoken, and a GATE message 606 and a REPORT message 608 are transmitted and received between the OLT and the ONU to thereby maintain the PON link between the OLT and the ONU while being in its sleep mode. Accordingly, the OLT needs to provide transmission and reception bandwidths also to the ONU in its sleep mode.
Note that the time intervals at which the OLT transmits a GATE message correspond to dynamic bandwidth allocation (DBA) intervals 612, as with the time intervals at which the OLT transmits a GATE message to active ONUs. Accordingly, the time intervals at which the ONU in its sleep mode is activated correspond to the DBA intervals.
Meanwhile, a PON using TDM and WDM in combination, i.e. TDM/WDM-PON, has been proposed, see U.S. patent application publication No. US 2011/0085795 A1 to Ozaki and Japanese patent laid-open publication No. 2011-135280, for example. There are TDM/WDM-PON systems in which the OLT has a plurality of optical subscriber units (OSUs), for example.
In a TDM/WDM-PON system, OSUs are allocated to transmission wavelengths different from each other. Each of the OSUs transmits a downstream optical signal on the transmission wavelength specifically allocated thereto. In turn, each ONU transmits an upstream optical signal on a transmission wavelength and timing indicated by the downstream optical signal sent by the OSU managing that ONU.
In the TDM/WDM-PON system, since communication is performed on wavelengths specific to the respective OSUs, the ONUs are adapted to have the wavelengths changeable on which they can transmit and receive signals. For that aim, the receiver of the ONUs is provided with a wavelength-tunable filter that can change its receivable wavelength on which downstream optical signals are conveyed. In addition, the transmitter of the ONUs is provided with a wavelength-tunable optical transmitter device that can change a wavelength on which it transmits upstream optical signals.
It is sufficient that each ONU is registered in any one of a plurality of OSUs. Therefore, in the TDM/WSM-PON system, a plurality of ONUs may distributively be managed among those OSUs. In such a case, the dynamic wavelength allocation (DWA) may be applied to the OLT to thereby allow each OSU to manage plural ONUs of which the number is variable depending on communication conditions, such as traffic conditions, see Tomoaki Yoshida, et al., “An automatic load-balancing DWBA algorithm considering long-time tuning devices for λ-tunable WDM/TDM-PON” ECOC 2013, We.2.F.5, NTT Access Network Service Systems Laboratories, NTT Corporation, for example.
In each OSU, a threshold is set for bandwidth which is to be allocated to ONUs, such bandwidth being also referred to as a bandwidth in use. The threshold is set to a value lower than the maximum bandwidth each OSU can allocate to the ONUs, the value being 80% of the maximum value, for example. The threshold is set either for bandwidth on which each OSU transmits downstream signals to ONUs or bandwidth on which it receives upstream signals from ONUs. In an OSU of interest, as traffic between the OSU and ONUs managed thereby increases, the bandwidth may exceed the threshold. In this case, DWA control may shift one or more of the ONUs managed by the OSU whose bandwidth in use exceeds the threshold to another OSU whose bandwidth in use is well below its threshold, which may be referred to as assignee OSU.
As a result of the DWA, the amount of traffic is substantially flattened, or load-balances, among the OSUs, resulting in increasing the service bandwidth provided to subscriber units. By the load-balancing, the TDM/WDM-PON system can as a whole maintain its quality of communication.
Note that ONUs of which the management is shifted by DWA to an assignee OSU will change their transmission and reception wavelengths specific to the assignee OSU.
With reference to FIGS. 2A and 2B, DWA will be detailed on a case where the cyclic sleep is applied to a TDM/WDM-PON. FIGS. 2A and 2B schematically illustrate how an OSU changeably allocates bandwidths in use to ONUs it manages. FIGS. 2A and 2B illustrate bandwidths in use before and after DWA, respectively. In the figures, the vertical axes represent bandwidth in use in arbitrary unit. Note that, in this example, the OSU manages a plurality (n) of ONUs, ONU-1 to ONU-n, including ONU-m rendered in sleep mode, where m is a natural number not exceeding a natural number n. The threshold 500 for the OSU is set to 80% of the maximum bandwidth 510 available for communication with the ONUs.
When the cyclic sleep is applied to a TDM/WDM-PON, each OSU transmits and receives a sleep permission signal and a sleep request signal, respectively, and periodically transmits and receives a GATE message and a REPORT message, respectively, to and from the ONUs which the OSU manages. Accordingly, the OSU managing an ONU got in sleep mode needs to prepare a bandwidth to be allocated to the ONU for use in transmitting a GATE message and receiving a REPORT message to and from that ONU. Consequently, that OSU suffers from deterioration in margin of bandwidth utilization caused by the allocation of the bandwidth to ONU-m staying in sleep mode, as illustrated in FIG. 2A. Thus, the total bandwidth 502 and 504 allocated to the ONUs exceeds the threshold 500 as shown in FIG. 2A.
In order to render the total bandwidth falling below the threshold 500, DWA is performed to shift the management of some of the ONUs to another OSU as illustrated in FIG. 2B. In this example, the management of two active ONUs, ONU-3 and ONU-4, is switched to another OSU.
In this way, application of the cyclic sleep to a TDM/WDM-PON involves the problem that allocation of a bandwidth to an ONU staying in sleep mode causes the margin of bandwidth utilization to decrease. As a result, the DWA may suffer from increase in the number of active ONUs of which the management is to be shifted to other OSUs.
As has been described above, ONUs whose management is shifted to other, i.e. assignee, OSUs change their transmission and reception wavelengths specific to the assignee OSUs. Active ONUs have to temporarily interrupt communication in order to prevent a frame or frames from being lost during a time period in which transmission and reception wavelengths are being changed. It would be possible that upstream and downstream signals that occur in a time period in which the transmission and reception wavelengths are being changed may be buffered until the wavelength has changed. Such interruption of communication and signal buffering would, however, lead to communication delay.